Euclides (archaic spelling Euclydes) da Cunha (January 20, 1866 – August 15, 1909) was a Brazilian writer, sociologist and engineer. His most important work is Os Sertões (Rebellion in the backlands), a non-fictional account of the military expeditions promoted by the Brazilian government against the rebellious village of Canudos, known as the War of Canudos. This book was a favorite of Robert Lowell, who put it above Tolstoy, the Russian writer.

Euclides da Cunha was also heavily influenced by Naturalism and its Darwinian proponents. Os Sertões characterised the coast of Brazil as a chain of civilisations while the interior was more primitively influenced.

Euclides da Cunha was the basis for the character of The Journalist in Mario Vargas Llosa's The War of the End of the World.